Conventional generating units use a generator in the form of a synchronous machine. Here, the electrical power is provided by the fixed generator part (stator). The shaft train driving the generator is identical to the shaft train of the drive, which is constructed in the form of a turbine, for example. The shaft train is in the generator part of a rotor with a rotor winding, which generates a magnetic excitation field, which rotates uniformly with the rotor and therefore with the speed of the drive. A rotary field magnetomotive force, which is stationary with respect to the rotating rotor, is thus produced in the generator, i.e. the rotary field magnetomotive force rotates at the same speed as the drive and the rotor. As the abovementioned shaft train (of the drive and of the rotor) rotates at network frequency in the case of a two-pole synchronous machine, for example, or at a fraction of the network frequency in the case of synchronous machines with a higher number of poles, the rotary field magnetomotive force is produced with a rotary field frequency of the same magnitude as the network frequency. That is to say, network-frequency alternating current is induced in the stator. However, because of this process, the speed of the shaft train is permanently coupled to the network frequency. This significantly restricts flexibility in the design of the torque-producing components—therefore of the drive, e.g. in the form of a gas, steam or water turbine or a piston engine or other drive unit.
To avoid this disadvantage, in the case of small generator powers, it has previously been possible to connect gearboxes between the drive and the generator. Other solutions provide for the whole outgoing power of the generator to be matched to the network frequency by electronic conversion by means of a network frequency converter. However, neither approach is suitable for high output powers. Gearbox solutions are conceivable up to powers of about 80 MW. Conversion of the full power of a generator has been realized up to about 25 MW depending on speed.
A further possibility consists in designing a generator working as a synchronous machine for an output frequency, which deviates from the required network frequency. In order to bring the output frequency to the network frequency, an electric motor, which rotates at network frequency or a fraction thereof, is operated directly by the generator and is driven by the generator in synchronous mode. For its part, the motor supplies network frequency. This procedure is capital-intensive and prone to losses, however, so that only unsatisfactory efficiencies can be achieved.
In summary, therefore, with high generator powers, the drive must necessarily rotate at network frequency or a fraction thereof, which constitutes an undesirable constraint when producing current by means of a generating unit.
It would be desirable to achieve a decoupling of drive and generator even at any predetermined network frequency.